Freeze-drying or vacuum-drying for foodstuff can provide a produce more excellent in quality than that by normal pressure heat drying, and therefore is used reputably as a means of process for preservation foodstuffs or space foodstuffs. However, there has been adopted in that process a surface heat drying system by a radiant heat source, so that if the radiant heat thereof is increased in thermal volume in order to shorten the drying time, the surface temperature of the foodstuff is raised to excess and cells of the foodstuff are dried hard or tissue thereof is damaged, and as a result the foodstuff is changed in color or flavor. Accordingly, it has been necessary to take the drying time of 6-10 hours for obtaining a food product good in quality.
In view of this, the inventors of the present invention have previously succeeded in shortening the drying time by using a microwave heating apparatus capable of heating a material to be heated from the central portion thereof, jointly with a radiant heat source. However, the conventional microwave heating apparatus is so arranged that an electric wave may radiate onto a material to be heated substantially perpendicularly thereto, so that there is involved such an inconvenience that an electric wave transmission distance through the material to be heated is short and the heating energy thereof is not effectively consumed. Additionally, the same has to provide in a heating chamber a heat generating plate or the like serving as a radiant heat source, so that there is involved such an inconvenience that an effectively available space in the heating chamber is small.